1. Field of the Invention
The present invention relates to a semiconductor device comprising a thin film transistor (hereinafter referred to simply as a xe2x80x9cTFTxe2x80x9d), and to a process for fabricating the same.
2. Prior Art
Thin film transistors have been applied heretofore to active-matrix addressed liquid crystal display devices or to image sensors. In particular, the active matrix liquid crystal display devices using TFTs as switching elements, i.e., those comprising TFTs being formed in each of the pixels, are attracting much attention as liquid crystal display devices capable of operating at high speed.
As is well known, light must be transmitted through the pixel portions of a liquid crystal display device. However, the electric conductivity of the active layer of a TFT increases as light is irradiated thereto, because the active layers arc made of a film of amorphous silicon or crystalline silicon which is usually photosensitive. The increase in conductivity of an active layer then unfavorably impairs the charge retention of a pixel electrode, because the off current, which is an important characteristic of a TFT switching element, is increased by the irradiation of light. Accordingly, it is necessary to establish a special constitution for the TFT, i.e., a constitution in which light is not irradiated to the active layer, and particularly, in which the channel-forming region of the active layer is isolated from light.
In the light of the aforementioned circumstances, the primary purpose of the present invention is to provide a structure in which a TFT is protected from light entering from outside in order to reduce an Ioff current of the TFT.
In accordance with a first aspect of the invention, a light-blocking (light shielding) film is interposed between a TFT and a light transparent substrate. More specifically, the light-blocking film comprises such a material which is capable of being changed to a light-transparent film upon oxidation or nitridation. Therefore, the unnecessary portion of the light-blocking film which is not located under the TFT but is selectively converted to light transparent without using a complicated photolithography process for pattering the light-blocking film.
Accordingly, the light blocking film prevents the TFT from being undesirably irradiated with light entering from a transparent substrate while the other portions of the light blocking film are changed to be transparent so that the device can operate as an electro-optical display device. The present invention is applicable not only to an active matrix liquid crystal device in which TFTs are provided on a pixel area of a transparent substrate or an image sensor, but also to an integrated circuit which employs TFTs formed on a transparent substrate.
In accordance with a first aspect of the invention, the light-blocking layer in the present invention comprises a material selected from the group consisting of silicon and aluminum, for example. These materials are light-shielding but can be made transparent upon oxidation or nitridation. Further, it is preferable to dope the silicon with an appropriate impurity, for example, phosphorous, arsenic, boron, aluminum or other IIIb or Vb group elements.
In accordance with another aspect of the invention, a semiconductor island which forms source, drain and channel regions therein has its peripheral portion nitrided or oxidized. Thereby, it is possible to prevent a leak current or short circuit which tends to occur between a gate electrode and the channel region through a gate insulating layer at side edges of the semiconductor island due to a stepped configuration of the semiconductor island.
The present invention will be described below in more detail in conjunction with the attached figures. It should be understood, however, that the present invention is not to be construed as being limited thereto.